Cargo within an airplane cargo deck is typically supported by a system of freely rotating floor-mounted conveyance rollers. Sets or banks of power drive units (PDUs) can be simultaneously elevated from beneath the cargo deck to a level just above the conveyance rollers. Spring-lift PDUs having surfaces which remain above the cargo deck may also be present in longitudinal areas of the cargo deck. Regardless of its type, each PDU may be a separate electro-mechanical actuator which includes one or more rubber coated wheels or drive rollers. The drive rollers of the elevated PDUs contact and move cargo above the conveyance rollers in the commanded direction upon energization. The movement of cargo depends on the coefficient of friction between the PDU drive rollers and the bottom surface of the cargo, as well as the lifting force generated by the PDU lift mechanism. When the PDUs are de-energized, roller rotation ceases and the cargo stops moving. Several sets of PDUs can be arranged along a common path of conveyance, and each set can be operated separately, thereby allowing for the transfer of multiple pieces of cargo. Loading personnel can guide cargo by means of a joystick, typically in combination with other switches, buttons and/or similar controls. Prior art PDUs, such as those disclosed in U.S. Pat. No. 5,661,384 and U.S. Pat. No. 7,014,038 are provided with an IR sensor to detect the presence of an ULD directly above a corresponding PDU.
FIG. 1 illustrates a layout of a cargo aircraft 100 having a cargo loading system 102 in accordance with the prior art. FIG. 2 presents an exemplary prior art wiring arrangement of the cargo loading system 102 connecting the principal elements of FIG. 1.
The aircraft 100 has a main cargo door 110 through which cargo containers and pallets (collectively known as unit load devices, or “ULDs”) enter and exit the main cargo deck 112 within the main cargo compartment 113. Installed on the main cargo deck 112 are a plurality of PDUs 114a, 114b, such as those mentioned above. In the aircraft shown, most of the PDUs are arranged in rows 116, 118, on either side of a longitudinal centerline C of the cargo deck 112. As seen in FIG. 1, the PDUs 114a are arranged in the right row 116 while the PDUs 114b are arranged in the left row 118. ULDs in the form of cargo containers loaded into the cargo aircraft 100 typically are arranged in two rows, one on either side of the centerline C. It is understood that such cargo containers on a given side are propelled in the longitudinal direction by the PDUs located on that side. Additional PDUs may be found in an omni-directional area 120 proximate the main cargo door 110. It is understood that the main cargo deck 112 of a typical cargo aircraft will also be provided with various non-powered rollers, guides, restraints and the like, none of which are shown, for simplicity.
In a large aircraft, there may be several dozen PDUs 114a, 114b. To control the PDUs, the cargo loading system is provided with a number of features. Among these are a controller, sometimes called a “main controller unit” (MCU) 130, a main control panel (“MCP”) 132, and a plurality of local control panels (“LCPs”), including control panels 134a on right side 136 of centerline C as you face the front of the aircraft 100, and control panels 134b on left side 138 of centerline C.
The MCU 130 comprises one or more processors physically connected to the PDUs 114a, 114b, the main control panel (“MCP”) 132, and the plurality of local control panels (“LCPs”) 134a, 134b. The connections between the various components may be made by one or more communication buses, each comprising one or more wires, cables, optical fibers or the like. As seen in FIG. 2, one possible bus configuration is that a first bus 140 connects the MCU 130 to the main control panel 132; a second bus 142a connects the MCU 130 to the right-side PDUs 114a and the right-side local control panels 134a; and a third bus 142b connects the MCU 130 to the left-side PDUs 114b and left-side local control panels 134b. 
The MCU 130 receives commands entered via the main control panel 132 and the local control panels 134a, 134b. In response to such commands, the MCU 130 sends appropriate signals to selected PDUs 114a, 114b, all in a manner known to those skilled in the art.
The MCP 132 is usually mounted next to the main cargo door 110. In one embodiment, the MCP 132 is mounted on a wall of the main cargo compartment 113, inside the aircraft. In some prior art aircraft, the MCP, or an auxiliary main control panel, may be accessible from outside the aircraft, at a point next to the main cargo door 110, allowing an operator outside the aircraft to control loading and unloading. In general, the MCP 132 includes indicator lights, a display comprising a screen and/or one or more rows of LEDs, and also such things as a joystick, buttons and/or switches to control the PDUs 114a, 114b. 
The LCPs 134a, 134b are mounted on the inside wall of the cargo compartment 113, at spaced intervals along the length thereof. The LCPs 134a, 134b are similar to the MCP 132, but do not usually include a display; their primary function is to activate the rollers of a power drive unit to propel an ULD forward or backward, in a particular row 116 or 118. As is known to those skilled in the art, when an LCP 134a, 134b is activated, it sends a first signal to the MCU 130. The MCU, using information about the location of the ULDs, determines which PDUs 114a, 114b in the rows 116, 118 should be turned on, and sends a second signal to one or more of the PDUs 114a, 114b. 
During loading operations, two or three person cargo loading/unloading teams are common. The primary operator runs the MCP 130 to move ULDs into and out of the main cargo door 110 from the off-aircraft loading platform. The assistant operator(s) utilize the LCPs 134a, 134b to control travel of cargo towards and away from the main cargo door 110 door, from within the longitudinal aircraft cargo compartment 113. If two assistant operators are present, each is commonly assigned a given side of the aircraft cargo compartment 113 to load. As is known to those familiar with air cargo operations, an ULD must be parked with sufficient precision so that it can be secured by cargo latches (not shown). This requires the assistant operators to be able to see the positions of each ULD well enough to be able to park it in a location which permits latching. And despite having numerous LCPs 114a, 114b, the fixed location of each on the wall of the cargo compartment 113 requires the assistant operators to stand at one of several discrete locations, providing them with less than ideal viewing of the required work area in which the ULDs are to be precisely placed.